US 6459596 B1 Abstract A method and apparatus for a multilevel rectifier with fewer switching components is provided. The method reduces the required number of switching devices in an n level switching device and clamp diode rectifier. The method is valid for any positive integer number of levels equal to or greater than three. The method is also applicable to any phase system. The method reduces the number of switching devices required for an n-level rectifier from the standard 2(n−1) by 2 switching devices for every phase leg. A multilevel uni-directional power converter system including a multilevel rectifier that has a reduced number of switching devices is provided. The system comprises an input to a multilevel rectifier with at least one phase leg. The rectifier is composed of switching device and anti-parallel diode pairs and clamping diodes. The number of switching devices required depends on the given number of voltage levels according to the equation 2(n−1) where n is the number of voltage levels. This number of required devices is then reduced in accordance with the present invention by removing the top and the bottom switching devices from the rectifier circuit. The anti-parallel diodes remain in the rectifier circuit. There is no performance degradation from this reduction in the number of switching devices to the rectifier.
Claims(6) 1. A reduced parts count multilevel rectifier comprising:
at least one input node;
at least one phase leg corresponding to each said input node;
a plurality of switching device and anti-parallel diode pairs arranged in each said phase leg according to a given number, n, greater than or equal to 4, of voltage levels wherein the number of said switching devices is reduced by removing two switching devices from the circuit but the anti-parallel diodes remain;
at least one clamping diode corresponding to each said remaining switching device and anti-parallel diode pairs;
a plurality of output nodes corresponding to the number of voltage levels; and
a plurality of capacitors connected between adjacent said output nodes.
2. A reduced parts count multilevel rectifier as in
3. A reduced parts count multilevel rectifier as in
4. A method for reducing the number of switching devices required in a multilevel rectifier for a given number of voltage levels comprising:
(a) choosing a number, n, of voltage levels desired, where n is at least 4;
(b) choosing the number of phase legs for the rectifier;
(b) designing a standard switching device and clamping diode rectifier based on said n voltage levels and number of phases;
(c) reducing the number of switching devices required by removing two switching devices from said rectifier.
5. A multilevel converter system comprising:
at least one input node;
a multilevel rectifier with at least one phase leg corresponding to and operably connected with each said input node, wherein said multilevel rectifier has fewer switching devices in each said phase leg than a standard multilevel rectifier for a given number of voltage levels, wherein said multilevel rectifier has at least four said voltage levels;
a plurality of rectifier output nodes corresponding to and operably connected with each of said multilevel rectifier phase legs;
a plurality of capacitors corresponding to and operably connected with each of said rectifier output nodes;
a multilevel inverter with at least one phase leg corresponding to and operably connected with each of said capacitors and rectifier output nodes;
at least one load output node corresponding to and operably connected with each said phase leg of said inverter; and
control means for said rectifier and said inverter.
6. A method for reducing the number of switching devices required in a multilevel rectifier for a given number of voltage levels comprising:
(a) choosing the number of phase legs for a multilevel rectifier;
(b) choosing a number, n, equal to or greater than 4, of voltage levels desired;
(c) calculating the number of switching device and anti-parallel diode pairs required for each said phase leg as 2(n−1);
(c) calculating the number of capacitors required as n−1;
(d) designing a standard switching device and clamping diode rectifier circuit for each said phase leg; and
(e) removing switching devices from the top and bottom of each said phase leg of said rectifier circuit while leaving said anti-parallel diodes in said rectifier circuit, whereby a reduced switching parts-count multilevel rectifier is achieved.
Description This application claims the benefit of U.S. Provisional Application No. 60/226,218 filed Aug. 18, 2000, entitled “Reduced Parts-Counts Multilevel Rectifier,” incorporated herein by reference. The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefore. This invention relates to multilevel switching component rectifiers, and more particularly to a multilevel rectifier with fewer switching components than known multilevel rectifiers. The invention is valid for any number of phases and at least three levels. The general trend in power electronics is to switch power semiconductors at increasingly high frequencies in order to minimize harmonics and reduce passive component sizes. However, the increase in switching frequency increases switching losses, which become significant at high power levels. Two methods for decreasing switching losses, and at the same time improving power quality, that have been proposed are constructing resonant converters and multi-level converters. Resonant converters avoid switching losses by adding an LC resonant circuit to the hard switched inverter topology. The inverter transistors can be switched when their voltage or current is zero, thus mitigating switching losses. Examples of this type of converter include the resonant DC link, and the Auxiliary Resonant Commutated Pole inverter (ARCP). One disadvantage of resonant inverters is that the added resonant circuitry will increase the complexity and cost of the converter control. Furthermore, high IGBT switching edge rates can create switch level control problems. Multi-level converters offer another approach to providing high power quality. One of the primary advantages of multilevel converters are the high number of switching states offered so that the output voltage can be “stepped” in smaller increments thereby producing better voltage waveforms. This allows mitigation of harmonics at low switching frequencies thereby reducing switching losses. In addition, EMC concerns are reduced through the lower common mode current facilitated by lower dv/dt's produced by the smaller voltage steps as well as reducing the switching dv/dt stresses allowing for potentially longer component life. One significant disadvantage of these techniques is that they require a high number of switching devices thereby increasing the cost and complexity of these circuits. Although the devices are rated at a lower voltage, gate drive and control circuitry must still be provided. Another disadvantage of multi-level inverters is that they must be supplied from isolated DC voltage sources or a bank of series capacitors with balanced voltages. In systems where isolated DC sources are not practical, capacitor voltage balancing becomes the principal limitation for multi-level converters. One possible solution to the voltage balancing problems inherent in multilevel converters is described in U.S. Pat. 6,031,738, Lipo et al. U.S. Pat. No. 5,644,483, Peng et al., which is hereby incorporated by reference, describes a multilevel converter system. The number of switching devices needed in the converter is determined by the equation m An example of a three-level rectifier with reduced numbers of switching devices was presented in Y. Zhao, Y. Li, and T. A. Lipo, “Force Commutated Three Level Boost Type Rectifier,” Although not disclosed by Zhao et al., the reduced parts count topology in FIG. 1A may be extended to rectifiers with a higher number of voltage levels such as the four-level topology illustrated in FIG It is desirable to provide a multilevel rectifier with a reduced number of switching devices to reduce the cost and complexity of converter systems with a high number of voltage levels and phases that is suitable for high voltage applications. A multilevel uni-directional power converter system including a multilevel rectifier that has a reduced number of switching devices is provided. The system comprises an input to a multilevel rectifier with at least one phase leg. The rectifier is composed of switching device and anti-parallel diode pairs and clamping diodes. The number of switching devices required depends on the given number of voltage levels according to the equation 2(n−1) where n is the number of voltage levels. This number of required devices is then reduced in accordance with the present invention by removing the top and the bottom switching devices from the rectifier circuit. The anti-parallel diodes remain in the rectifier circuit. There is no performance degradation from this reduction in the number of switching devices to the rectifier. The clamping diodes separate the switching devices from the output nodes. The output nodes are joined through a group of series connected capacitors that serve as the input source for a multilevel inverter. The multilevel inverter has the requisite number, 2(n−1), of switching devices for the given number of voltage levels. The multilevel inverter has a set of outputs that supply the conditioned power to a load. Control of the switching for the rectifier is achieved through hysteresis current control and redundant state selection with feedback from the capacitor bank. Control of the multilevel inverter is achieved through redundant state selection regulated by duty-cycle modulation. Additionally a method for reducing the required number of switching devices in a multilevel rectifier is provided for any given number of levels and phases. The method comprises designing a traditional n-level rectifier, where n is the number of voltage levels, and then removing the top and bottom switching devices from each phase leg of the rectifier circuit. This method does not result in any appreciable performance loss for the circuit. An important design consideration is that this method will only work for uni-directional power flow (AC to DC). Additional advantages of the invention will be set forth in the description which follows, and will in part be obvious from the description and drawings, or may be learned from practicing the invention. FIG. 1A is a prior art three level reduced parts count rectifier. FIG. 1B is an extension of the prior art three level reduced parts count rectifier to four levels. FIG. 2 is a block diagram of a multilevel converter system incorporating a reduced parts count multilevel rectifier in accordance with the invention. FIG. 3 is a schematic diagram of a prior art fully active four level rectifier/inverter. FIG. 4 is a schematic representation of the equivalent switch states for a four-level inverter. FIG. 5A is a schematic diagram of one phase leg of a three level reduced parts count rectifier in accordance with the invention. FIG. 5B is a schematic diagram of one phase leg of a four level reduced parts count rectifier in accordance with the invention. FIG. 6 is a graph of an example reference current and a-phase current for a four-level rectifier utilizing current hysteresis control. FIG. 7 is a group of functional schematic diagrams of the cases depicting the redundant switching states for a four level rectifier. FIG. 8 is a vector plot of the rectifier voltage vectors for a reduced-parts count three-level rectifier. FIG. 9 is a vector plot of the rectifier voltage vectors for a reduced-parts count four-level rectifier. FIG. 10 is a vector plot of the rectifier voltage vectors for a reduced-parts count six-level rectifier. FIG. 11 shows waveform diagrams depicting the steady-state performance of a fully active four-level rectifier. FIG. 12 shows waveform diagrams depicting the steady-state performance of a reduced parts count four-level rectifier in accordance with the invention. FIG. 13 shows waveform diagrams depicting the performance of a fully active four-level rectifier under a step change in load. FIG. 14 shows waveform diagrams depicting the performance of a reduced parts count four-level rectifier in accordance with the invention under a step load. Reference to the drawings will be made to describe the preferred embodiments of the present invention. FIG. 2 is a functional block diagram of a converter system. The fixed frequency AC source Herein, the term converter will be used generically to describe any power electronic conversion device. Specific terms such as rectifier or inverter are used to specify a particular converter. U.S. Pat. No. 4,270,163, Baker, taught how to achieve multilevel inverters of four or more levels by adding switching devices with anti-parallel diodes in pairs on an upper and lower end of the phase leg along with clamp diodes. FIG. 3 shows one topology for a four-level inverter A fully active four-level rectifier FIGS. 5A and 5B illustrate the preferred embodiment of one phase leg of a three and four level reduced parts count rectifier according to the present invention. As can be seen there is a savings of two switching devices Table 1 illustrates the semiconductor savings for a three-phase system.
As can be seen, the switching device In the preferred four-level topology of the present invention, all of the switching devices An 18 kW laboratory test system with the structure shown in FIG. 2 was constructed for rectifier
The induction motor where x represents the phase and may be a, b, or c. The switching state s In the rectifier where S Voltages (6-8) are defined from the rectifier phases to the neutral connection of the source. Duty-cycle modulation control where V
The electrical angle can be related to a desired electrical frequency by It can be seen from (1), that the PWM switching has direct control of the inverter line-to-ground voltages v where v
and m is the modulation index having a range of It is often convenient to define a modulation index that has a range from 0 to 100% by The motor phase voltage resulting from the commanded line-to-ground voltages can be determined by substituting (14-16) into (2-4) yielding By comparing (20-22) to (9-11), it can be seen that the commanded voltages are obtained if the modulation index is set to PWM switching is accomplished by defining duty-cycles based on the normalized commanded line-to-ground voltages, which may be expressed as The inverter
where INT is the integerization function which will return the nearest integer less than or equal to its argument. The nearest switching states are then s where
The b- and c-phase switching states are computed in a similar manner. Typically, the switching frequency is set to roughly one hundred times the fundamental frequency (1/T The multi-level hysteresis current-regulator For the tested rectifier
When the current error is positive, the controller In order for the four-level power conversion processes (rectifier and inverter) to operate properly, the voltages on all three capacitors All cases discussed above can be analyzed off-line and programmed as a table into an erasable programmable read only memory (EPROM) or programmable logic device (PLD). Based on the desired switching state (s The overall DC link voltage v where θ
and
where e
The inverse transformation necessary to determine i All necessary transformation terms can be determined from these terms using trigonometric identities. Harmonics in the line voltages will appear in the sensor outputs, but may eliminated using a low-pass filter in hardware or software. Compensation for the filter amplitude attenuation and phase delay can be incorporated in the control With respect to reduced parts count rectifiers FIGS. 5A and 5B, the primary concern is the limitation on performance due to the reduced number of switching devices
Consider the case where the current vector lies between the dashed lines as shown in FIG. In order to determine the limitation imposed on the reduced parts count rectifier, consider the case of the four-level reduced parts count rectifier with the voltage vector plot shown in FIG. Expressing this limitation as a function of the commanded current is helpful. This is accomplished by transforming the rectifier input voltage and inductance to the utility reference frame. In the steady-state, these equations become
In the utility reference frame, v _{d} ^{u}=0 (45)
Since unity power factor is commanded, the d-axis utility reference frame current is set to zero and the q-axis current is regulated to a negative value as per (35-36). Using this information, and transforming the rectifier voltages to the stationary reference frame yields a d-axis voltage of As can be seen from FIG. 9, the worst case operating condition, in terms of violating the voltage constraint, is when θ Equation (47) is used to evaluate the suitability of the four-level reduced parts count rectifier under the specific operating conditions given above. Notice that the limit is increased with increasing commanded DC voltage and decreasing input voltage. Furthermore, lower values of line inductance and voltage source frequency will increase the limit on q-axis current. Based on the parameters and operating conditions of the example system mentioned above, it is determined from (47) that the magnitude of the q-axis current must be limited to 93.2A if a reduced parts count rectifier is used. With this current limitation and unity power factor operation, the rectifier input power is limited to 48 kW, which is more than enough for the 18 kW test load. FIGS. 11 and 12 show the system steady-state performance for the fully active rectifier and reduced parts count rectifier respectively. Therein, the motor phase voltage v FIGS. 13 and 14 demonstrate the system performance under a step change in load for the fully active rectifier and reduced parts count rectifier respectively. In the test system, the motor load What has been described is only a few of many possible variations on the same invention and is not intended in a limiting sense. The claimed invention can be practiced using other variations not specifically described above. Patent Citations
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